Diel Activity Rhythms of the Rodents, Microtus Ochrogaster and Sigmodon Hispidus Hispidus

In the vicinity of Nashville, Davidson County, Tennessee there occur two species of rodents which frequent the same or neighboring fields. These two species are the prairie meadow mouse, Microtus ochrogaster (Wagner), and the cotton rat, Sigmodon hispidus hispidus Say and Ord. In their major adjustments to the community of which they are a part these two rodents seem to be ecologically equivalent (Allen, 1870-71; Bailey, '31). In the more southern latitudes of the United States Sigmodon replaces Microtus. Sigmodon is a much larger animal and, therefore, constructs larger surface and underground runways. The adult weight of Sigmodon is approximately 125 grams whereas that of Microtus is 45 grams. According to Hamilton ('43) the above area lies at the extreme southern range of Microtus ochrogaster and the extreme northern range of Sigmodon hispidus hispidus in the eastern United States. We find that the opinions of field workers based on trapping data vary greatly in regard to the time of day at which Microtus is most active. A diurnality was found by Bailey ('31) for M. mexicanus in New Mexico and by Jackson ('08) for M. pennsylvanicus in Wisconsin. However, both Kennicott ('56) and Lyon ('36) found M. pennsylvanicus to be mainly nocturnal. Johnson ('30), Bailey ('26), and Goodwin ('35) state that M. pennsylvanicus is arhythmic, although there is a tendency for an increase in activity at dusk and during the early morning hours. A similar situation appears to exist in regard to cotton rats. Aldrich and Bole